This invention relates to an optical image sensing device having an improved optical shield for an array of sensing elements, to reduce effectively the "shading effect" of the array, and more particularly, it is applicable to an infrared image sensing device (hereinafter referred to as an IR detector).
In an IR detector the impinging infrared energy is converted to electrical energy proportionally by the array of semiconductor sensing elements (hereinafter referred to as sensing elements), which works well at extremely low temperatures such as 77.degree. K. These sensing elements of the array are arranged in a line, or over an area whose shape is usually a square or a rectangle. The IR energy emitted from the object to be observed is focused on the array by optical collecting lenses, and filtered through an infrared filtering window. The focused IR image is converted to electrical signals and processed with an electronic apparatus to be displayed on a displaying device such as a screen of a cathode ray tube.
The IR emission or radiation varies sharply with the temperature of the object. Usually, the detectable temperature difference between the object and its background, or between a portion of the object that is of interest and another portion of the object, is expected to be small, such as 0.1.degree..about.0.2.degree. C. Recently, the sensitivity of the IR sensing elements has been improved remarkably, and the sensitivity of the individual elements has become sufficiently uniform to extract a clear image of such an object. As the result, the improvement of the signal/noise ratio has become a serious problem to be solved to obtain a high grade image. One of the sources of noise is extraneous photons intruding from outside the field of view (hereinafter denoted by FOV) of the sensing array. For the purpose of reducing the noise, an optical shield is set in front of the IR array to shield it from the extraneous IR energy and to reject it by restricting the solid angle of the FOV.
To obtain a high grade image, a uniform distribution of the impinging IR energy to each IR element is necessary as a matter of course, but with a prior art shield, this problem has been left unsolved. This problem is called "shading", which usually occurs in the marginal portion of the image.
Generally, the solid angle of the FOV of the array is defined by the ratio of the size of the aperture to the distance between the IR array and the shield. The prior art optical shield has a single aperture through which the incoming IR energy is projected, and the size of the aperture is large. Therefore, the distance between the IR array and the shield becomes large, in proportion to the size of the aperture. This does not allow a prior art IR detector to be miniaturized.
The material of prior art IR sensing elements is an IR sensing semiconductor such as mercury-cadmium-telluride, whose operating temperature is very low as stated above, so that the array is attached with low heat resistance to a base plate of a high thermal conductivity material such as copper, and insulated thermally from the outside environment by a vacuum chamber (Dewer vessel). The array is cooled through the base plate by a refrigerator arranged inside the Dewer vessel.
Sometimes this optical shield is called a cold shield, because the shield should be kept cold to reduce the IR radiation from the shield itself, which might provide the IR image with another source of noise.
The IR energy is emitted from an object to be measured, is collected by a positive lens and filtered through an IR filtering window, and impinges onto the surface of the array, after the FOV is restricted by the shield aperture.
As shown in FIG. 1 in perspective, the optical shield 7 has a single aperture 3 (opening window), with its shape depending on the shape of the array. For example, for a linear array 1 of IR elements 2, a rectangular aperture 3 is provided. The sides 3a of the rectangular window 3 that are parallel to the linear array 1 shield the sensing elements effectively from the extraneous photons outside the FOV angle, but the other sides 3b, which are transverse to the linear array 1, do not shield the array 1 effectively. It is clear that the distribution of the impinged IR energy onto the elements 2 is not uniform along the axis line X--X of the linear array, but has a peak at the center and is reduced somewhat at both sides. This is the shading referred to above. Naturally, this will result in an IR image with speckles and a partially faded background. As described before, the temperature difference to be detected is very small, so that such a noise and shading will affect the image quality of the IR detector seriously.
A counter-measure is proposed in U.S. Pat. No. 3,963,926, issued June 15, 1976, wherein an array of individual optical shields fabricated by semiconductor technology is disclosed. The advantage of this prior art is that, with this type of shield, each IR sensing element is surrounded by a shielding section of the shielding array, so the shielding itself is performed individually. This means the number of sensing elements is exactly the same as the number of the optical shield elements. By such a shielding, the shading is eliminated almost completely.
But, as the semiconductor technology progresses rapidly, the pitch of the sensing elements on the array is becoming closer, such as several tens of microns, so there remains little space between the sensing elements to set the individual shields surrounding each element. In other words, the pitch of the sensing elements of the IR array is limited by its individual shield. This results in setting a restriction on the resolving power of the IR impinging apparatus.